First identification, observation under the electron microscope, and reproduction of photomicrographs of bacteriophages taken through the electron microscope.

Digital facsimile, but with poor reproductions of the two plates, from PubMedCentral at this link.

This work demonstrated the absorption of the phages on the host cell, and the lysis of the host cell and the liberation of a hundred or so daughter paritcles from each cell. It also showed that the particles multiply inside the cells rather than at their surfaces, since unti lysis occurs the number of particles visible at the surface remains constant. 
Digital facsimile from PubMedCentral at this link. For an extensive commentary on this paper, including correspondence between Anderson and Delbruck before and after publication see T. F. Anderson, "Electronic microscopy of phages," Phage and the origins of molecular biology. Expanded Edition, edited by John Cairns, Gunther Stent and James D. Watson (1992) 63-78.

In 1969 Luria shared the Nobel Prize in Physiology or Medicine in with Delbrück (No. 2578.5) and A. D. Hershey (No. 256) "for their discoveries concerning the replication mechanism and the genetic structure of viruses."

Order of authorship in the original publication: Luria, Human. Luria and Human discovered the restriction modification system found in bacteria and other prokaryotic organisms. This system provides a defense against foreign DNA such as that carried by bacteriophages. They found that bacteriophage growing within an infected bacterium could be modified, so that upon their release and re-infection of a related bacterium the bacteriophage’s growth is restricted (inhibited). 

"It was found that, for a bacteriophage λ that can grow well in one strain of Escherichia coli, for example E. coli C, when grown in another strain, for example E. coli K, its yields can drop significantly, by as much as 3-5 orders of magnitude. The host cell, in this example E. coli K, is known as the restricting host and appears to have the ability to reduce the biological activity of the phage λ. If a phage becomes established in one strain, the ability of that phage to grow also becomes restricted in other strains" (Wikipedia article on Restriction enzyme, accessed 3-2020). 

Digital facsimile from PubMedCentral at this link.